1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a printer, using an electrophotographic or electrostatic recording system and to a developing apparatus to be used in the image forming apparatus.
2. Related Art
Nowadays, an increase in speed, a reduction in size, an improvement in image quality, and an increase in service life are required of image forming apparatuses. In addition, with the development of the information technology, there is an increasing demand for color image output.
An example of a technique for meeting the demand for an increase in speed is a tandem type image forming apparatus in which developing apparatuses for four colors are arranged laterally or longitudinally. Despite the fact that a plurality of developing apparatuses are arranged, no increase in apparatus size is involved since each developing unit is thin. FIG. 13 shows an example of such an image forming apparatus.
FIG. 13 is a side sectional view showing the inner construction of a tandem type printer. As shown in the drawing, a sheet conveying belt 111 is arranged substantially at the center of the interior of the apparatus. The belt 111, whose end portions are held by a driving roller 112 and a driven roller 113, circulates counterclockwise as indicated by an arrow A. On top of the belt 111, there are arranged four photosensitive drums 114 (114a, 114b, 114c, and 114d) in a sheet conveying direction (from the right to the left in the drawing).
On the upstream side of the belt 111 with respect to the sheet conveying direction, there is provided a registration roller pair 124 for correcting skew feeding of a sheet being fed. On the upstream side thereof, a sheet guiding path 125 is provided, and on the upstream side thereof, a sheet feeding roller 126 is provided. A sheet cassette 104 is arranged on the upstream side of the sheet feeding roller 126.
(Description of a Process Cartridge in an Image Forming Apparatus)
FIG. 14 is an enlarged sectional view of a process cartridge 134 to be mounted in the printer of FIG. 13. The process cartridge 134 is obtained by forming two sub units: a drum unit 135 and a developing unit 136 into an integral unit. The drum unit 135 is equipped with the photosensitive drum 114 mentioned above, a cleaner 115 arranged on the left-hand side thereof, and a charging roller 116 arranged above and on the left-hand side of the photosensitive drum 114. Light in accordance with image information emitted from a scanner 117 mounted in the printer main body impinges upon the photosensitive drum 114 at a position indicated by reference numeral 137.
The photosensitive drum 114 consists of a conductive metal roller whose surface is uniformly coated with a photoconductive material; the metal roller portion is grounded, and rotates clockwise as indicated by an arrow C.
The charging roller 116 is connected to a high-voltage power source and applies a blank-state charged potential (initialization potential applied uniformly; in a system it is applied in positive polarity, in another in negative polarity) to the photosensitive drum 114, whereby the photosensitive layer on the peripheral surface of the photosensitive drum 114 is uniformly charged, for example, in negative high potential and initialized.
The scanner 117 is equipped with a laser light source or an LED light source, and performs selective exposure in accordance with image information on the peripheral surface of the photosensitive drum 114 charged in negative high potential. As a result of this exposure, there is formed on the surface of the photosensitive drum 114 a negative low potential portion with attenuated potential, and there is formed an electrostatic latent image consisting of the low potential portion and the high potential portion obtained through the initialization.
In the developing unit 136, a non-magnetic toner is stored so as to substantially fill a developing container 118, and a toner agitating member 144 is arranged so as to be buried under the toner; in the lowermost portion, a toner supply roller 145 formed of a sponge material is held in press contact with a developing roller 143. A developing blade 146 in the form of a plate spring is held in press contact with the upper right portion of the developing roller 143.
(Description of the Operation of the Image Forming Apparatus)
The operation of the above-described image forming apparatus will be briefly described with reference to FIGS. 13 and 14. First, when a power source is turned on and instructions on the property and number of the sheets to be used, printing mode and the like are input through keys or as signals from a host apparatus connected to the image forming apparatus, one of the sheets accommodated in the sheet cassette 104 is supplied to the registration roller pair 124 by way of the sheet feeding roller 126. The rotation of the registration roller pair 124 is temporarily stopped, and the conveyance of the sheet is on standby with the leading end of the sheet abutting the nip portion defined by the registration roller pair 124.
Subsequently, the driving roller 112 rotates counterclockwise, and the driven roller 113 also rotates counterclockwise, whereby the entire belt 111 circulates counterclockwise, with its upper portion in contact with the four photosensitive drums 114.
At the same time, the developing containers 118 and the photosensitive drums 114 are successively driven in synchronism with printing. The photosensitive drums 114 rotate clockwise, and the charging rollers 116 impart a uniform high negative charge to the peripheral surfaces of the photosensitive drums 114; the scanner 117 performs exposure on the peripheral surfaces of the photosensitive drums 114 in accordance with an image signal to form low potential portions, whereby electrostatic latent images are formed which consist of high negative potential portions due to initialization and low negative potential portions due to exposure. The developing rollers 143 of the developing containers 118 transfers toner to the low potential portions of the electrostatic latent images to thereby form toner images on the peripheral surfaces of the photosensitive drums 114 (reversal development).
As the forward end of the toner image on the peripheral surface of the upstream-end photosensitive drum 114a approaches an opposite portion of the belt 111, the registration roller pair 124 starts rotation such that the printing start position of the sheet is matched with the opposite portion, and the sheet is fed to the sheet inlet portion.
The driven roller 113 and an attraction roller (not shown) convey the sheet while holding it with the belt 111. The sheet is attracted by the belt 111 and conveyed to a first transfer portion formed by the photosensitive drum 114a and a transfer roller 119a. 
The transfer rollers 119 (119a, 119b, 119c, and 119d) apply transfer current output from a transfer bias power source (not shown) to the sheet through the belt 111. By the transfer current applied from the transfer rollers 119, the toner images on the photosensitive drums 114 are transferred to the sheet.
The sheet to which the toner images in four colors have been transferred is separated from the belt 111 and is conveyed to a fixing apparatus 128. The fixing apparatus 128 fixes the toner images to the sheet by heat and pressure. After the image fixation, the sheet is discharged onto a discharge tray 106 by a discharge roller pair 132 with the toner image facing downwards.
However, as shown in FIG. 14, in the process cartridge 134, the toner supply roller 145 is provided in the lowermost portion of the developing unit 136, so that the toner is carried and supplied downward from above, and the stress on the developing unit 136 is rather large, which makes the toner subject to deterioration.
In the developing unit 136, there is provided the developing roller 143 arranged opposite to the photosensitive drum 114, the developing roller 143 carrying and conveying toner on its surface.
The developing roller 143 is opposed to the developing blade 146. The toner supplied onto and carried by the developing roller 143 hits the developing blade 146, whereby the thickness of the toner layer is regulated.
Thus, the toner in the vicinity of the developing blade 146 is under pressure due to the formation of the toner layer. In addition, a large pressure due to compression of the toner in the gravitational direction is applied thereto. As a result, the toner in the vicinity of the developing blade 146 is not circulated, and suffers stress due to the friction with the developing blade 146 and the toner supply roller 145, which makes the toner subject to deterioration.
In view of this, it might be possible to arrange the developing roller 143 above a toner storage chamber. In such a construction, the toner in the vicinity of the developing blade 146 would not undergo compression due to the gravitational action, and the toner would not easily deteriorate.
However, since it is a construction in which toner is supplied upwards from below, it is rather difficult to secure stability in the toner supply to the developing roller 143.
The present invention has been made in view of the problem described above in the related art. It is an object of the present invention to provide a developing apparatus in which toner does not easily deteriorate and an image forming apparatus equipped with such a developing apparatus.
Another object of the present invention is to provide a developing apparatus in which toner does not easily deteriorate and in which toner is supplied to a developing roller in a stable manner and an image forming apparatus equipped with such a developing apparatus.
Still another object of the present invention is to provide a developing apparatus including:
a housing;
a developer bearing member rotatably provided at an opening of the housing;
a developer containing room;
a carrying member for carrying the developer in developer containing room toward the developer bearing member; and
a developer accumulating portion arranged near the developer bearing member and adapted to accumulate the developer carried by the carrying member,
wherein the developer accumulating portion is provided above a rotational center of the carrying member.
Still another object of the present invention is to provide an image forming apparatus including:
an image bearing member;
developing means for developing an electrostatic latent image formed on the image bearing member; and
transfer means for transferring a developer image on the image bearing member to a recording material, the transfer means being arranged above the image bearing member,
wherein the developing means includes:
a housing;
a developer bearing member rotatably provided at an opening of the housing;
a developer containing room;
a carrying member for carrying the developer in developer containing room toward the developer bearing member; and
a developer accumulating portion arranged near the developer bearing member and adapted to accumulate the developer carried by the carrying member,
wherein the developer accumulating portion is provided above a rotational center of the carrying member.